【安卓笔记】HandlerThread源码剖析

有时候我们需要在应用程序中创建一些 常驻的子线程不定期地执行一些计算型任务 ,这时候可以考虑使用HandlerThread,它具有 创建带消息循环的子线程 的作用。

一、HanderThread使用示例
先熟悉下HandlerThread的一般用法。我们创建一个如下所示的Activity:
package com.example.handlethreaddemo;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.Looper;
import android.os.Message;
import android.util.Log;
public class MainActivity extends Activity
{
	private Looper mLooper;
	private MyHandler mHandler;
	private static final String TAG = "MainActivity";
	private static class MyHandler extends Handler
	{
		public MyHandler(Looper looper)
		{
			super(looper);
		}
		@Override
		public void handleMessage(Message msg)
		{
			switch (msg.what)
			{
			case 1:
				Log.i(TAG, "当前线程是"+Thread.currentThread().getName()+",TEST 1");
				break;
			case 2:
				Log.i(TAG, "TEST 2");
				break;
			}
		}
	}
	
	@Override
	protected void onCreate(Bundle savedInstanceState)
	{
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		//创建HandlerThread对象
		HandlerThread myHandleThread = new HandlerThread("HandlerThread<子线程>");
		//启动HandlerThread---->内部将启动消息循环
		myHandleThread.start();
		//获取Looper
		mLooper = myHandleThread.getLooper();
		//构造Handler,传入子线程中的Looper
		mHandler = new MyHandler(mLooper);
		/*
		 * 注:经过上述步骤,Handler将绑定子线程的Looper和MessageQueue.
		 * 也就是说handleMessage最终由子线程调用
		 * */
		mHandler.sendEmptyMessage(1);
		
		Log.i(TAG,"当前线程是:"+Thread.currentThread().getName());
		
	}
	
	
}

使用HandlerThread内部提供的Looper对象构造Handler对象,然后在ui线程中向Handler发送消息。log日志如下:


可见发送消息的线程为UI线程,而处理消息的线程为子线程,也就是说,我们在 子线程中创建了消息循环
一般情况下,我们总是在UI线程中创建Handler对象,并使用界面组件提供的默认Looper,这个Looper绑定在UI线程上。所以我们在线程中向Handler发送消息时,最终的处理是在主线程中进行的。但正如开篇所说,我们有时需要构建常驻的子线程以不定期执行计算型任务,这时在子线程中创建消息循环将非常有用。

二、HandlerThread源码剖析
HandlerThread源码十分精简。
HandlerThread继承自java.lang.Thread,并封装了Looper对象:
int mPriority;//优先级
 int mTid = -1;//线程标志
 Looper mLooper;//消息循环

可通过构造器注入线程优先级,默认优先级为Process.THREAD_PRIORITY_DEFAULT
 public HandlerThread(String name, int priority) {
        super(name);
        mPriority = priority;
    }

核心逻辑为run方法(复写Thread类的run方法):
 public void run() {
        mTid = Process.myTid();
        Looper.prepare();//创建Looper对象
        synchronized (this) {
            mLooper = Looper.myLooper();//获取与本线程绑定的Looper
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();//回调接口,默认为空实现。
        Looper.loop();//启动消息循环--->may be blocked
        mTid = -1;
    }

外界可通过getLooper方法获取Looper对象:
 public Looper getLooper() {
        if (!isAlive()) {//线程死亡
            return null;
        }
        
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) {
                try {
                    wait();//异步等待Looper准备好
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }

如果调用getLooper方法时,Looper未准备好,那么将会阻塞线程,直到准备好Looper对象。
外界可调用quit方法终止消息循环:
 public boolean quit() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quit();//内部调用looper类的quit
            return true;
        }
        return false;
    }

附:
Looper类相信大家都不陌生,这里顺便简单提下(之前写过Handler和Looper):
Looper.prepare方法将会创建一个Looper对象(Looper类的构造器为私有,不可new),并将其放到ThreadLocal中,意为线程局部变量:
public static void prepare() {
        prepare(true);
    }
    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

然后通过Looper.myLooper方法返回与本线程绑定的Looper,正是刚创建的Looper:
public static Looper myLooper() {
        return sThreadLocal.get();
    }

Looper.loop方法将启动消息循环,不断从其内部封装的消息队列MessageQueue中取出消息,交由Handler执行。

public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;
        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();
        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }
            // This must be in a local variable, in case a UI event sets the logger
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }
            msg.target.dispatchMessage(msg);
            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }
            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }
            msg.recycle();
        }
    }

没有消息时,消息队列会阻塞。

以上就是HandlerThread的全部内容。




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